Optical elements having tunable optical characteristics by using the known electro wetting effect have been a highly attractive modulation scheme for a variety of optical applications. A large number of devices based on electro wetting tuning have been devised: For example, electro wetting has been used as an optical switch for fiber optics, as an optical shutter or filter for camera and guidance systems, in optical pickup devices, in video display devices, and in optical waveguide materials.
In the present document the term ‘liquid cell’ describes a single optical element for tuning the propagation of a bundle of light beams. The liquid cell includes a chamber having cell walls filled with at least two different non-miscible liquids, especially a conductive polar liquid, like water, and an insulating non-conductive liquid, like oil.
An ‘interface area’ separates the insulating, non-conductive liquid from the polar liquid.
A tuning voltage fed to control electrodes moves a ‘surface area’ that describes the border of the cell wall surface to the adjacent polar liquid. Changing the tuning voltage causes the changing of the surface area by adjusting the contact angle between the surface area and the cell walls. The contact angle is the angle at which a liquid interface area meets a solid surface. The solid surface is formed by the cell walls (in the liquid cell).
The term ‘electro wetting effect’ defines the change of said contact angle due to an applied potential difference between the solid and the polar liquid.
Recently liquid cells forming tunable optical elements are known from several publications. For instance, the published US-document no. US2006/079728, (Kuiper et al.) describes an apparatus for providing a fluid meniscus with variable configurations by means of electro wetting. The apparatus comprises a single fluid chamber that holds two different fluids separated by an interface area of which the edge, having different sides, is constrained by a closed set of adjacent cell walls of a fluid chamber. A first control electrode acts on a first side of the meniscus edge and a second control electrode acts separately on a second side of the meniscus edge. Selected meniscus configurations can be formed by providing different tuning voltages to both electrodes respectively.
The apparatus published in document US2006/079728 describes an optical apparatus that allows desirable fluid meniscus configurations, which are not rotationally symmetric about the optical axis of the apparatus. For example, configurations which are tilted and/or astigmatic about the optical axis may be provided. A range of fluid meniscus configurations can be formed in a variable, controllable manner. Exemplary meniscus configurations include flat shapes and anamorphic lens shapes capable of the precise refractive or reflective angular deflection of light in up to three dimensions. The meniscus configurations can be formed accurately and efficiently under the application of variable voltage patterns applied across the configuration of electrodes and a common steering electrode.
The document US2006/079728 does not disclose a device comprising millions of arrayed tunable optical micro cells that are individually addressable and controllable for tuning by electro wetting. Further, the document does not disclose how to manufacture this device avoiding a high effort of manufacturing technology.
An essential disadvantage of conventional liquid cell products such as a liquid Fresnel lens or planar cell array having long rectangular liquid cell is that it will not work well, because the long rectangular channel geometry of such prior art cannot support a stable and accurate tunable interface between the adjacent liquids.
Current research activities on the fields of imaging, displays, and communications have shown, there is a need for a flat optics comprising individually and continuously tunable liquid cells arranged in a matrix to enable a full implementing of the refractive power of classical ‘glass’ optics.
These flat optics are necessary, for instance, to realize automatic stereoscopic displaying of two different video images or for reconstructing holographical three-dimensional scenes at different places of an observer volume. An additional application of these flat optics can be used to steer laser beams for applications such as laser radar. Furthermore, such flat optics can be used to direct solar energy for lighting or electricity generation.
As described in the above-mentioned document electro wetting devices voltage-modulate the contact angle between a polar liquid and a dielectric to tune the interface area between two non-miscible liquids. The contact angle modulation can then be used to reconfigure, for instance, a saline meniscus geometry and therefore to chance the refraction angle or to chance the phase of light. Electro wetting devices modulate the contact angle between a saline liquid and a dielectric by a tuning voltage at control electrodes and corresponding counter electrodes. This contact angle modulation can then be used to reconfigure the interface area geometry between the adjacent different fluids and therefore to modulate incident light waves. Each cell of the flat optic device comprises cell wall electrodes being arranged essentially parallel directed to the optical axis of the optical array device. The optical axis of the array device and/or of a single liquid cell is oriented essentially perpendicular to the surface of the supporting substrate.
An optical flat array device having discrete tunable liquid cells can be used for multiple application of spatial light modulation; for instance to create, individual beam deflection functions in a light wave field or spatial light modulator means realizing phased-array function, when the incident light is coherent.
More details referring to the principle of electro wetting are described in the references:
L. Hou, N. Smith, and J. Heikenfeld, Electrowetting Modulation of Any Flat Optical Film, Appl. Phys. Lett., Vol. 90, 251114, 2007
J. Heikenfeld, N. Smith, D. Abeysinghe, A. J. Steckl, and J. W. Haus, Flat Electrowetting Optics, IEEE LEOS Newsletter, Vol. 20 (4), pp. 4-10, August 2006. and
B. Sun, K. Zhou, Y. Lao, W. Cheng, and J. Heikenfeld, Scalable Fabrication of Electrowetting Pixel Arrays with Self-Assembled Oil Dosing, Appl. Phys. Lett., Vol. 91, 011106, 2007.
Furthermore, it is highly desirable to have a means for economic manufacturing and arraying millions of liquid cells having cross-sections of some ten micrometers to a stable array having a high density and a high light transfer. The liquid cells should be discretely controllable by electric signal means, like video signals or computer graphic means for displaying video content two or three dimensional or reconstructing moving three dimensional scenes by means of holographic technique. Prior art provides no means by which such large arrays of devices which can be manufactured at the cost and precision required for commercial holographic displays or wide-aperture laser radar.
Therefore, it is an object of the invention to provide and manufacture a tunable optical array device that comprises liquid cells realizing variable and spatial light beam steering of incident light, which continuously varies the optical characteristics of the interface area preferably with a lateral precision below one micro-meter.
According a further object of the invention the tunable optical array device should be useful in spatial light modulator means to realize spatial phase-shifting in transmitted portion light waves capable for temporal or spatial interference.
It is further an object of the invention to create a method that uses techniques to allow on-demand manufacturing of high-aspect-ratio cell structures with lateral precision below one micro-meter.
A further object of the invention is to reduce the tuning voltage to drive the control electrode system that tunes the interface areas in the liquid cells to voltages below 15V. That has the advantage that tuning and cell addressing circuitry can be used that is compatible to the conventional circuitry of Thin-Film-Transistor-Technology for easy adoption in the manufacturing process of Liquid Crystal Display devices using straight forward conventional photo resist and coating process technology.
A further object of the invention is to provide a manufacturing technology for a tunable optical array device comprising liquid cells that is useful for easy manufacturing high aspect ratio cells, having an improved aspect ratio and an extended fill factor to constitute efficient transfer of light into the desired directions.
As used herein, the term high aspect-ratio means: the walls separating the liquid cells of the array have a geometric structures that are much taller than wide and or thick. The same used technology should provide means for manufacturing both two-liquid or three-liquid cells easy to fill with liquids.
A further object of the invention according to an exemplary embodiment is to constitute a fast redirection and/or modulation of a complete light wave field fed by coherent light or modulated with holographic information to different outgoing directions more than some hundred changes per second, for instance in synchronization to a sequence of video frames for holographic reconstructing three-dimensional scenes by video holograms at different eye positions in an observer volume of a holographic reconstruction device.